The present invention relates to a porous ceramic structure suitably used, for example, as a filtration material of a filter.
There has been used a porous ceramic structure containing a ceramic excellent in thermal resistance and corrosion resistance as a filtration material for filters used for an environmental measure such as antipollution or the like, or recovery of products from high temperature gas, or the like, in various fields including chemistry, electric power, steels, and industrial waste disposal. A honeycomb-shaped porous ceramic structure has suitably been used as a dust-collecting filter used at high temperature in a corrosive gas atmosphere, such as a diesel particulate filter (DPF), which traps particulate matter (PM) discharged from a diesel engine such as an automobile diesel engine.
As a porous ceramic structure used in a dust-collecting filter, there has widely been used a honeycomb-shaped porous ceramic structure 25 where a number of cells 23 are partitioned and formed by the partition walls 24, and the inlet side end face B and the outlet side end face C are alternately plugged with a plugging member 22, for example, a dust-collecting filter 21 shown in FIG. 3. According to the dust-collecting filter 21, which has such a structure, when the target gas G1 introduced in a part of the cells 23 from the inlet side end face B passes through the partition walls 4 and flows into the adjacent cells 23, particulate matter containing the target gas G1 is trapped in the partition walls 24. Then, treated gas G2, which has passed through the partition walls 24 and flew in the adjacent cells 23, is discharged from the outlet side end face C. Therefore, treated gas G2, where particulate matter in the target gas G1 is separated and removed, can be obtained.
By the way, in recent years, a porous ceramic structure having high porosity has been demanded because of the necessity of improving treatability of a duct-correcting filter by reducing a pressure loss when gas passes through partition walls. When such a porous ceramic structure having high porosity is manufactured, generally a combustible power which functions as a pore former is added to a forming raw material. The combustible powder is burnt off when the formed body is fired, and pores are formed at the location where the powder has been present. Therefore, a porous ceramic structure having high porosity can be obtained. As such a combustible power to function as a pore former, there have conventionally been used solid particles such as carbon, cokes, or solid resin particles, or hollow particles such as foaming resin particles (see Patent Documents 1 to 3).
Patent Document 1: JP-A-7-163822
Patent Document 2: JP-A-2003-40687
Patent Document 3: JP-A-2004-315346
However, in the case that solid particles among the above combustible particles are used as a pore former, the particles are hardly smashed upon mixing/kneading of a forming raw material because the particles are solid, and therefore, a stable porosity can be secured. On the other hand, since the particles have high exothermic rate upon firing, the particles have a problem of frequently having inferiority such as a crack and an internal defect. In the case that hollow particles are used as a pore former, the particles have low exothermic rate upon firing since the particles are hollow, thereby inhibiting inferiority as described above from being caused. However, since the particles are easily smashed upon mixing/kneading of a forming raw material, a stable porosity cannot be secured, and the particles have a problem of deteriorated filter properties (pressure drop for trapping, strength, trapping efficiency, etc.), for example, when the particles are used for a filtration material for a filter.
The present invention has been made in such conventional circumstances and aims to provide a method for manufacturing a porous ceramic structure, the method being capable of inhibiting the combustible powder functioning as a pore former from being smashed upon mixing/kneading a forming raw material and suppressing an excess heat generation upon firing and manufacturing a porous ceramic structure having a stable porosity with a good yield.